Wow, I remember the disarm, but I didn't see what happened. Looking at the video, it looks like he just was trying to wind around it quickly, so it was probably just very unexpected, resulting in lost grip.
BTW, Brian's "best 2 out of 3" fights for 3rd place start 1:52:20 in the video (link below will go to the right time index). It looks like the audio is horribly out of sync. Plus the video gets jerky at time. Hopefully we'll see more individual-fight videos get posted as well.
https://youtu.be/v9JPvavIOAM?t=1h52m20sEdit: FYI-- As I stated above, the fights were "2 out of 3", so when you see Brian win the first bout, keep watching. It gets really intense, down to the end of the final bout with tied score, and multiple "sudden deaths".
(that's not a spoiler, anymore than the thread's title is... lol)